1. Field of the Invention
This invention relates to rotational couplings and, in particular, to a wrap spring clutch having features to assist disengagement of the wrap spring upon release of the clutch.
2. Discussion of Related Art
A conventional wrap spring clutch includes an input hub and an output hub. A wrap spring is coupled to one of the input and output hubs by an interference fit. When rotation of the hub to which the wrap spring is connected in a first rotational direction causes the spring to tighten down upon the other hub thereby coupling the two hubs together for rotation and engaging the clutch. The spring is unwrapped, and the clutch released, when the hub to which the wrap spring is coupled stops or reverses direction or as the result of an external force that prevents rotation of the wrap spring.
The operation of a wrap spring clutch depends on differences in rotational speed among the input and output hubs. In particular, the wrap spring will engage and disengage more quickly when there are large difference in rotational speed among the hubs. In many applications, however, the differences in rotational speed during engagement and disengagement of the clutch are relatively low. As a result, the wrap spring may cause an undesirable drag on the hub on which it has been wrapped during clutch disengagement and generate unwanted noise and vibration. The wrap spring is also susceptible to increased wear as a result. Wrap spring clutches are therefore often unsuitable for applications in which the differences in rotational speed among the hubs are relatively low. Conventional wrap spring clutches attempt to overcome these issues by adjusting the interference fit of the wrap spring to the hubs. Adjusting the interference fit, however, requires a delicate balance to permit both clutch engagement (which can be compromised if there is insufficient interference) and disengagement (which can be compromised if there is too much interference). In many applications, a proper balance cannot be obtained and conventional wrap spring clutches cannot be used.
The inventors herein have recognized a need for a rotational coupling that will minimize and/or eliminate one or more of the above-identified deficiencies.